A class of bacteriophage T4rII mutants was observed some years ago, in a collection induced by gamma irradiation, that reverted at very high rates; stocks could contain on the order of 0.1% of revertants. Contrary to expectations based on previous work in other organisms, as well as in T4, these mutants did not contain duplications of substantial length, but instead behaved like point mutants, or at least mutants comprising no more than a few base pair changes. A much larger collection of such mutants has been found among spontaneous and gamma-induced rII mutants, and subjected to genetical analysis to determine their nature. Virtually all of these mutants turn out to arise at highly redundant DNA sequences composed of ...AAAAAA..., and probably to consist of the addition of one A to the series.